In recent years, the chopper circuit has been widely employed in the various kinds of control arrangements in order to control the DC voltage to be applied to a load. A chopper circuit normally includes switching elements for controlling the current to be supplied to the load, and the other elements such as external commutation means for turning on and off the switching elements. It is particularly advantageous to be able to reduce the number of switching elements and to enhance the performance of the circuit. Particularly, in a chopper circuit employing a thyristor as the switching element, when the forward current whose value i.sub.o flows into the thyristor, it is necessary to render the resultant current flowing through the thyristor zero by supplying a reverse-current whose value is at least i.sub.o, in order to turn off the thyristor. As the thyristor, however, is not always turned off by rendering the resultant current zero, a commutation capacity for obtaining a reverse-current value of more than i.sub.o is required in the chopper circuit.
Conventional and prior art chopper arrangements are shown in FIGS. 1 and 2. Referring particularly to FIG. 1, a chopper circuit CH comprise a main thyristor 10, a first auxiliary thyristor 12 connected in parallel through a commutation capacitor 14 to the main thyristor 10 and a second auxiliary thyristor 16 (which may be replaced by a suitable semiconductor such as a diode) connected in series across a commutation reactor 18 to the thyristor 12. The chopper circuit CH is connected to a direct current power source such as a battery 20 in series relationship through a electric motor 22. The motor 22 consists of an armature winding 24, a field winding 26 and a free-wheeling diode 28 which is connected in parallel with a series circuit consisting of the armature winding 24 and the field winding 26.
In the arrangement of FIG. 1, the chopper circuit CH can control a relatively great load current. In the circuit, however, it is necessary to provide three switching elements, that is, the main thyristor 10, the first auxiliary thyristor 12 and the second auxiliary thyristor 16, as is shown in FIG. 1, and accordingly the arrangement is complicated and expensive.
FIG. 2 shows a known chopper circuit which consists of a switching element such as, for example, a main thyristor 10, a series circuit consisting of a reactor 18 and a capacitor 14, and a series connected diode 30 which can be omitted. In the circuit of FIG. 2, the only one thyristor is employed and, therefore, the cost of the circuit is lowered in comparison with the chopper circuit of FIG. 1.
The waveforms shown in FIG. 3 are illustrative of the mode of operation of the FIG. 2 circuit. In operation, when the thyristor 10 in FIG. 2 is turned on as occurs at time t=t.sub.o, the initial load current I.sub.B whose value is i.sub.o flows into the thyristor 10. The load current value i.sub.o is assumed to be constant as a matter of convenience, when the thyristor 10 is in one state. On the other hand, the currents I.sub.C (I.sub.C1 and I.sub.C2) designated in FIG. 2 as flowing through the capacitor 14 try to flow symmetrically in both directions, because its polarity is inverted at time t=t.sub.1 as shown by the waveform l.sub.c of FIG. 3. Accordingly, the current I.sub.S flowing through the thyristor 10 is the resultant current (I.sub.C + I.sub.B) which is the sum of the capacitor current I.sub.C and the current I.sub.B flowing through the load, as shown the curve l.sub.s.
In this case, if the peak value i.sub.cp of the current I.sub.C is smaller than the value i.sub.o, the thyristor current I.sub.S would not become zero even if the reverse current I.sub.C2 with respect to the current I.sub.C1 tries to cancel the load current I.sub.B, and therefore the thyristor 10 will never be turned off. Accordingly, to turn off the main thyristor 10, it is necessary that the peak value i.sub.cp of the capacitor current I.sub.C be greater than the value i.sub.o of the load current I.sub.B. However, when the current I.sub.S flowing through the main thyristor 10 becomes zero at the time t.sub.3 illustrated in FIG. 3, the capacitor current I.sub.C is superimposed on the load current I.sub.B when the main thyristor 10 is in ON state, and, as a result, the peak value of the thyristor current I.sub.S becomes greater than twice the value i.sub.o. Therefore, since the rated capacity of the thyristor 10 is required to be greater than the twice of the load current value, the switching element itself becomes costly.
In like manner, as the only one thyristor is used in the chopper circuit of FIG. 2, the number of circuit elements may be few and the treatment of low current can be performed economically. However, in case of treating a greater current, the circuit of FIG. 2 lacks the commutation capability, and is expensive because a greater capacity and high price capacitor must be used in order to compensate the shortage of commutation capacity.
Further prior art circuit arrangements are disclosed in U.S. Pat. No. 3,365,640 and U.S. Pat. No. 3,555,399 wherein each of the circuit arrangements, respectively, includes a first switching element, a series circuit connected in parallel to said first switching element, and consisting of an inductance element, a capacitance element and a diode for preventing the reverse current, and a second switching element connected in parallel to said diode and said inductance or capacitance element.
The circuit arrangements described above are basically and substantially different from the circuit arrangement of the present invention.